Multifunctional alcohols including glycols, glycerols and higher polyhydric alcohols as components in the preparation of polymers such as polyesters, polyurethanes, polycarbonates, polyethers and the like are widely used in the resins and plastics industry. A variety of glycols are commonly used for preparing linear polymers and as chain extenders to increase molecular weights or to introduce `block` segments in polymers to impart specific properties.
Appropriate functional groups in functionalized diols can improve properties of polymers prepared from them. For example, U.S. Pat. No. 4,248,994 teaches use of polyols, containing at least one carboxamide group in the polyol backbone, in the preparation of polyurethane resins. These resins can be formed in the fluid state and show accelerated curing characteristics without adversely affecting pot life of the resin or the physical properties of the cured resin. U.S. Pat. No. 4,107,151 discloses use of a polyol, containing at least one urethane bond, in the preparation of a urethane elastomer. The elastomers exhibited short mold separation period, high initial strength after gelation, lack of phase separation, and no significant sacrifice in pot-life or chemical stability.
Urethane or carbamate containing diols, where one or more carbamate groups link the two hydroxy functions, are well known in the art. These diols are prepared by an extension of the method of ring opening reaction of cyclic carbonates with amines as taught in U.S. Pat. Nos. 2,627,524 and 3,703,538. U.S. Pat. No. 3,248,373 describes the reaction of diamines with two equivalents of cyclic carbonates to give bis(betahydroxyalkyl)carbamates useful as polyurethane chain extenders. U.S. Pat. Nos. 4,126,747 and 4,161,596 disclose similar procedures for making carbamate diols which are intermediates for olefinic monomers. U.S. Pat. Nos. 3,595,814 and 4,177,342 teach use of the same aminoalcohols with cyclic carbonates. U.S. Pat. No. 4,500,717 describes ring opening of a cyclic carbonate with a preformed carbamate monoalcohol to prepare carbamate diols.
Polyols, in particular diols, containing a urethane or carbamate group in the side chain are less well described. These side chain carbamate monomers have the added advantage that during long term usage of the polymers therefrom, any hydrolytic cleavage of the carbamate group would not significantly alter molecular weights of the polymers, and mechanical integrity of devices prepared from them would be sustained. Such monomers have been described in U.S. Pat. No. 2,928,812 as components for condensation with formaldehyde and urea to give water dispersible polymers. However, hydroxyalkyl carbamates described therein are again prepared by the ring opening of glyceryl carbonate with ammonia and are limited to N-unsubstituted carbamates.
Asymmetric substitution pattern of glyceryl carbonate also results in a mixture of two possible carbamate diols, 2,3-dihydroxypropyl carbamate and 2-hydroxy-1-hydroxymethyl ethyl carbamate by the above synthetic method. This method requires a tedious and expensive separation of isomeric products to obtain pure isomers. U.S. Pat. No. 4,177,342 extended this concept by using amines instead of ammonia to give N-substituted carbamate diols, further usable as components for polyurethanes. Disadvantages of asymmetric substitution obtained from cyclic carbonates again resulted in mixtures of isomeric products from the ring opening reaction as mentioned above. Use of such an isomeric mixture of monocarbamate diols in polymerization results in randomized spacing of side chain carbamate groups along the polymer backbone and randomly removes spacer alkylene groups between the side chain carbamate function and the polymer backbone. The effect of both of these factors is to reduce intramolecular interactions between the NLO active carbamate substituents.
Another procedure that has been taught for making branched urethane diols involves the non-stoichiometric reaction of a triol with a monoisocyanate. U.S. Pat. No. 4,107,151 describes the reaction of one equivalent of a triol with one equivalent of a monoisocyanate to form monocarbamate diols. This method has limitations both from lack of regiospecificity, giving a mixture of possible monocarbamate diols, and from possibilities of forming di- and tri-carbamates in the mixture. Use of such a mixture in polymerization reactions seriously reduces molecular weights and causes premature gelling. This method can also prove inadequate in the present invention for preparing pure monocarbamate diols, especially as a nonlinear optically (NLO) active comonomer. A slight amount of cross-linking can severely inhibit poling the resulting polymer to make a useful NLO material.
The present invention teaches a generalized synthesis of carbamate diols which provides pure single isomers, useful in polymers, some of which in turn are useful in nonlinear optics.
Laser techniques have been developed so that it is possible to obtain a limited number of fundamental frequencies of coherent laser light by utilizing solid, gas, and liquid media. However, in many applications, laser light having frequencies not among the fundamental frequencies obtainable is required, and in some cases laser light exhibiting a continuous spectrum over a certain range of frequencies is required. Nonlinear optical crystals have, therefore, frequently been employed to convert coherent laser light of a fundamental frequency into laser light of the second harmonic, that is to say, laser light with a frequency twice the fundamental frequency.
U.S. patents relating to nonlinear optical properties of organic materials include U.S. Pat. Nos. 4,774,025, 4,779,961, and 4,818,899.
Recent publications relating to nonlinear optical properties of organic materials include T. Kurihara et al., "A New Organic Material Exhibiting Highly Efficient Phase-matched Second Harmonic Generation: 4-Methoxy-4'-nitrotolan", J. Chem. Soc., Chem. Commun., 959-960, (1987); and J. Mori and T. Kaino, "Molecular Orbitals of Benzoxadiazole Compounds with Optical Nonlinearities", Physics Letters A, 127, pp 259-262 (1988).